Help Me Choose

Systems Explained

The three most common boiler types in the UK include Combi Boilers, System Boilers and Regular Heat Only Boilers.

Combi Boilers

House Combi System Diagram

 System Boilers

House System Diagram

Heat Only Boilers

House Diagram

Combi Boilers

‘Combi’ is short for combination boiler. As the name suggests, a combi boiler will provide hot water and central heating. Combi boilers are generally best suited to small-medium sized properties with a maximum of 20 radiators and up to 2 showers (dependant on KW).

Please note: If you wish to use both showers at the same time, a standard combi boiler would not be suitable. A stored combi boiler (combi with stored water) or system boiler & cylinder would be bested suited to your needs.

How Combi Boilers Work

With a combi boiler you won’t have any stored hot water as combi boilers heat up the water as you draw from your hot outlets.

Combi boilers connect directly to your water main, the mains water pressure pushes the water through a plate heat exchanger located within the boiler to transfer the heat generated by the main heat exchanger.

Like how the hot water is produced, the heat for the central heating system is produced in the same way. The water passes through a separate section of the plate heat exchanger and is circulated out to the radiators via an internal pump.

A diverter valve located within the boiler box will compress or depress according to your selection to provide either central heating or hot water.

Suitability for Your House

Number of radiatorsNumber of showersRecommended KW rating
Up to 101 Bathroom24-27kw
Up to 151 Bathroom & en suite28-34kw
Up to 201 Bathroom & en suite35-42kw

Note: A combi boiler would not be recommended for properties comprising of more than 1 bathroom & en suite and with 20+ radiators (or if you plan to use 2 showers at a time). Alternative systems such as a system boiler & unvented cylinder would be more suitable.

Combi Boilers – Pros and Cons

Efficient – efficiency ratings of 90%+No back up if fault occurs – if your combi breaks down, there’s no immersion you can use as a backup.
Compact – everything is built into the box so no external components.Limited use – you generally only use one hot outlet at any one time, for example; if someone is using the shower and someone else opens a hot tap the flow will drop.
Cheaper to install – all in one means easier installation compared to older systems.
Pressure – hot water delivered at the same pressure as mains.
Instant – no waiting around for your cylinder to heat back up as combi boilers heat on demand.

System Boilers

System boilers are very similar to conventional boilers in that they both work on the principle of stored hot water (cylinder).

The key difference between the two systems is that the heating and hot water components have been built into the system boiler. System boilers are sealed systems which work alongside cylinders (external to boiler) to provide heating and hot water to a premises. Recommended for medium to large sized houses with two or more shower outlets and a higher demand for hot water.

How System Boilers Work

System boilers transfer the heat generated from the main heat exchanger to a plate heat exchanger where an internal pump will distribute to the central heating system and/or hot water cylinder. A big advantage about this type of system is that you can run more than one outlet at a time.

Brilliant for high hot water demandTake up more space than combi
Good water pressureHigher system  installation costs (boiler + Cylinder + control components)
EfficientLimited hot water – can only store set qty
No storage tank in loft
Compatible with solar panels

Sizing for system and regular boilers

System boilers and regular boilers preheat hot water which is stored in a hot water cylinder, therefore hot water flow rate is not as critical as it is with a combi boiler. The heat load for an average home can vary from around 10 kW to 20 kW, which means system boilers and regular boilers can be sized lower than combi boilers.

Regular Boilers

Regular or conventional/traditional boilers only produce heat and all other major components are external to the boiler (pump, expansion vessel etc). Regular boilers are found on open vented systems. Open vented systems consist of; boiler, cylinder, domestic storage tank (normally found in loft) and feed & expansion tank for central heating (again normally found in loft). Regular boilers tend to be found in older and larger homes, less and less are now being installed as homeowners move over to more efficient systems (sealed system boilers).

Choosing the right system

Don’t stress, if you are thinking of changing your system and not just your boiler, Boiler Monkey is on hand with qualified heating and gas safe engineers that can recommend and size up your new system. Start by filling out an online quote.

Get your new boiler installed from only £1595

Fill out our Free online quote form

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Here at Boiler Monkey Ltd we only work with the best brands in the industry, we will only fit boilers that meet our strict quality policy and boilers that are backed with comprehensive manufactures parts and labour warranties. Boiler manufactures we work with;

Vaillant Logo
Worcester Logo
Ideal Logo

Vaillant – The Vaillant Group is an international family-owned company with a heritage of more than 141 years in providing customers with energy-saving and easy to operate solutions for heating, cooling and hot water. Vaillant has been setting the standards in the heating market, creating products that have revolutionised the industry, from water heaters in the early 1900’s, to pioneering the ‘combi’ boiler over 35 years ago.

Vaillant Logo
Ideal Logo

Ideal – Ideal Boilers was founded in 1906, with over 100 years of providing the best quality boilers to their customers continually working to meet ever-changing domestic heating needs. Taking boilers from basements into kitchens in the 1920s, through to introducing pioneering high-efficiency technologies, Ideal boilers continually deliver reliable products designed with their customers in mind. Today, Ideal remains at the forefront of the domestic and commercial heating markets, leading the industry in setting new standards and challenging technological boundaries. Ideal boilers remains true to its founding principles of quality, innovation and value – building on its rich heritage as they look to the future.

Worcester – Originally founded in 1962, Worcester became part of the Worldwide Bosch Group in 1996 and have become the UK market leader in domestic boilers. Worcester’s commitment to the highest standards of manufacturing over the last 50 years, has led their boilers to receive coveted Which? Best Buy awards for nine years running, and for Worcester to receive a Royal Warrant for supplying their boilers to Her Majesty The Queen’s Sandringham Estate. UK headquarters are based in Worcester where all the main functions of the business are located including the manufacturing of gas boilers. Worcester has a further site in Clay Cross Derbyshire where oil boilers are manufactured.

Worcester Logo

Get your new boiler installed from only £1595

Fill out our Free online quote form

Start Now!

Manufacturers Warranties


Vaillant Logo
BudgetEcoFit Pure7 Year parts & labour
Mid RangeEcoTec Pro10 Year parts & labour
PremiumEco Tec Plus10 Year parts & labour


Ideal Logo
BudgetLogic +7 Year parts and labour
Mid RangeLogic Max10 Year parts and labour
PremiumVogue Max10 Year parts and labour


Worcester Logo
Mid RangeGreenstar I & RI5 Year parts and labour 
PremiumGreenstar CDi5 year parts and labour 

NOTE: all manufactures warranties listed above are subject to an annual boiler service over the full duration of the warranty period. Failure to uphold annual servicing will void the warranty. Servicing of the boiler must be carried out by a registered gas safe engineer and the engineer must complete the servicing section of the boilers benchmark book each time. Please refer to manufacturer links above for full details.